Fig. 8

Possible scenarios for the evolution of skeletal modularity in lamprey. For these results, we posit that the absence of type II collagen in the branchial arches in Petromyzon marinus is a derived rather than ancestral trait for lamprey. (A) The Semi-Differentiation Hypothesis states that lamprey mucocartilage is the result of halted differentiation wherein a semi-differentiated chondrogenic mesenchyme commits instead to the fibroblast lineage. In this scenario, absence of traditional soxD/E homologs permits the development of mucocartilage, as direct or indirect inhibition of gdf5/6/7 signaling promotes hyaline chondrogenesis. This scenario would partially explain the presence of multiple chondrogenic ECM genes in mucocartilage like type II and IX collagen. (B) The Direct Cooption Hypothesis states that the ancestral function of gdf5/6/7 signaling was primarily chondrogenic and was only later coopted in mucocartilage fibroblasts. This scenario would partially explain the diversity of mucocartilage-like phenotypes with respect to hyaline. (C) The Indirect Chondrification Hypothesis states that the ancestral gdf5/6/7 module was a generalist pathway involved in mesenchyme differentiation and only later acquired chondrogenic function. This scenario would partially explain the differences in lecticans between these tissues, as one was specific for mesenchyme and the other for developed hyaline